/*
 * Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.nio.file;

import java.nio.file.attribute.BasicFileAttributes;
import java.io.IOException;

/**
 * A visitor of files. An implementation of this interface is provided to the
 * {@link Files#walkFileTree Files.walkFileTree} methods to visit each file in
 * a file tree.
 *
 * <p> <b>Usage Examples:</b>
 * Suppose we want to delete a file tree. In that case, each directory should
 * be deleted after the entries in the directory are deleted.
 * <pre>
 *     Path start = ...
 *     Files.walkFileTree(start, new SimpleFileVisitor&lt;Path&gt;() {
 *         &#64;Override
 *         public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
 *             throws IOException
 *         {
 *             Files.delete(file);
 *             return FileVisitResult.CONTINUE;
 *         }
 *         &#64;Override
 *         public FileVisitResult postVisitDirectory(Path dir, IOException e)
 *             throws IOException
 *         {
 *             if (e == null) {
 *                 Files.delete(dir);
 *                 return FileVisitResult.CONTINUE;
 *             } else {
 *                 // directory iteration failed
 *                 throw e;
 *             }
 *         }
 *     });
 * </pre>
 * <p> Furthermore, suppose we want to copy a file tree to a target location.
 * In that case, symbolic links should be followed and the target directory
 * should be created before the entries in the directory are copied.
 * <pre>
 *     final Path source = ...
 *     final Path target = ...
 *
 *     Files.walkFileTree(source, EnumSet.of(FileVisitOption.FOLLOW_LINKS), Integer.MAX_VALUE,
 *         new SimpleFileVisitor&lt;Path&gt;() {
 *             &#64;Override
 *             public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs)
 *                 throws IOException
 *             {
 *                 Path targetdir = target.resolve(source.relativize(dir));
 *                 try {
 *                     Files.copy(dir, targetdir);
 *                 } catch (FileAlreadyExistsException e) {
 *                      if (!Files.isDirectory(targetdir))
 *                          throw e;
 *                 }
 *                 return CONTINUE;
 *             }
 *             &#64;Override
 *             public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
 *                 throws IOException
 *             {
 *                 Files.copy(file, target.resolve(source.relativize(file)));
 *                 return CONTINUE;
 *             }
 *         });
 * </pre>
 *
 * @since 1.7
 */

public interface FileVisitor<T> {

  /**
   * Invoked for a directory before entries in the directory are visited.
   *
   * <p> If this method returns {@link FileVisitResult#CONTINUE CONTINUE},
   * then entries in the directory are visited. If this method returns {@link
   * FileVisitResult#SKIP_SUBTREE SKIP_SUBTREE} or {@link
   * FileVisitResult#SKIP_SIBLINGS SKIP_SIBLINGS} then entries in the
   * directory (and any descendants) will not be visited.
   *
   * @param dir a reference to the directory
   * @param attrs the directory's basic attributes
   * @return the visit result
   * @throws IOException if an I/O error occurs
   */
  FileVisitResult preVisitDirectory(T dir, BasicFileAttributes attrs)
      throws IOException;

  /**
   * Invoked for a file in a directory.
   *
   * @param file a reference to the file
   * @param attrs the file's basic attributes
   * @return the visit result
   * @throws IOException if an I/O error occurs
   */
  FileVisitResult visitFile(T file, BasicFileAttributes attrs)
      throws IOException;

  /**
   * Invoked for a file that could not be visited. This method is invoked
   * if the file's attributes could not be read, the file is a directory
   * that could not be opened, and other reasons.
   *
   * @param file a reference to the file
   * @param exc the I/O exception that prevented the file from being visited
   * @return the visit result
   * @throws IOException if an I/O error occurs
   */
  FileVisitResult visitFileFailed(T file, IOException exc)
      throws IOException;

  /**
   * Invoked for a directory after entries in the directory, and all of their
   * descendants, have been visited. This method is also invoked when iteration
   * of the directory completes prematurely (by a {@link #visitFile visitFile}
   * method returning {@link FileVisitResult#SKIP_SIBLINGS SKIP_SIBLINGS},
   * or an I/O error when iterating over the directory).
   *
   * @param dir a reference to the directory
   * @param exc {@code null} if the iteration of the directory completes without an error; otherwise
   * the I/O exception that caused the iteration of the directory to complete prematurely
   * @return the visit result
   * @throws IOException if an I/O error occurs
   */
  FileVisitResult postVisitDirectory(T dir, IOException exc)
      throws IOException;
}
